Particle re-acceleration and Faraday-complex structures in the RXC\ua0J1314.4-2515 galaxy cluster

Radio relics are sites of electron (re)acceleration inmerging galaxy clusters but the mechanism of acceleration and the topology of the magnetic field in and near relics are yet to be understood. We are carrying out an observational campaign on double relic galaxy clusters starting with RXC J1314.4-2515. With Jansky Very Large Array multiconfiguration observations in the frequency range 1-4 GHz, we perform both spectral and polarization analyses, using the rotation measure (RM) synthesis technique. We use archival XMM-Newton observations to constrain the properties of the shocked region. We discover a possible connection between the activity of a radio galaxy and the emission of the eastern radio relic. In the northern elongated arc of the western radio relic, we detect polarized emission with an average polarization fraction of 31 % at 3 GHz and we derive the Mach number of the underlying X-ray shock. Our observations reveal low levels of fractional polarization and Faraday-complex structures in the southern region of the relic, which point to the presence of thermal gas and filamentary magnetic field morphology inside the radio emitting volume. We measured largely different RM dispersion from the two relics. Finally, we use cosmological magnetohydrodynamical simulations to constrain the magnetic field, viewing angle, and to derive the acceleration efficiency of the shock. We find that the polarization properties of RXC J1314.4-2515 are consistent with a radio relic observed at 70 degrees with respect to the line of sight and that the efficient re-acceleration of fossil electrons has taken place

Particle acceleration in a nearby galaxy cluster pair: the role of cluster dynamics

Context. Diffuse radio emission associated with the intracluster medium (ICM) is observed in a number of merging galaxy clusters. It is currently believed that a fraction of the kinetic energy in mergers is channeled into nonthermal components, such as turbulence, cosmic rays, and magnetic fields, which may lead to the formation of giant synchrotron sources in the ICM.Aims. Studying merging galaxy clusters in different evolutionary phases is fundamental for understanding the origin of radio emission in the ICM.Methods. We observed the nearby galaxy cluster pair RXC J1825.3+3026 (z similar to 0.065) and CIZA J1824.1+3029 (z similar to 0.071) at 120-168 MHz with the LOw Frequency ARray (LOFAR) and made use of a deep (240 ks) XMM-Newton dataset to study the nonthermal and thermal properties of the system. RXC J1825.3+3026 is in a complex dynamical state, with a primary ongoing merger in the E-W direction and a secondary later stage merger with a group of galaxies in the SW, while CIZA J1824.1+3029 is dynamically relaxed. These two clusters are in a pre-merger phase.Results. We report the discovery of a Mpc-scale radio halo with a low surface brightness extension in RXC J1825.3+3026 that follows the X-ray emission from the cluster center to the remnant of a galaxy group in the SW. This is among the least massive systems and the faintest giant radio halo known to date. In contrast to this, no diffuse radio emission is observed in CIZA J1824.1+3029, nor in the region between the pre-merger cluster pair. The power spectra of the X-ray surface brightness fluctuations of RXC J1825.3+3026 and CIZA J1824.1+3029 are in agreement with the findings for clusters exhibiting a radio halo and clusters where no radio emission has been detected, respectively.Conclusions. We provide quantitative support to the idea that cluster mergers play a crucial role in the generation of nonthermal components in the ICM

Fractional polarization as a probe of magnetic fields in the intra-cluster medium

Context. It is now established that magnetic fields are present in the intra-cluster medium (ICM) of galaxy clusters, as revealed by observations of radio halos and radio relics and from the study of the Faraday rotation measures of sources located either behind or within clusters. Deep radio polarization observations of clusters have been performed in the last years, and the properties of the ICM magnetic field have been constrained in a small number of well-studied objects. Aims. The aim of this work is to investigate the average properties of the ICM magnetic fields, and to search for possible correlations with the ICM thermal properties and cluster radio emission. Methods. We have selected a sample of 39 massive galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample, and used Northern VLA Sky Survey data to analyze the fractional polarization of radio sources out to 10 core radii from the cluster centers. We have investigated how different magnetic field strengths affect the observed polarized emission of sources lying at different projected distances from the cluster center. In addition, statistical tests are performed to investigate the fractional polarization trends in clusters with different thermal and non-thermal properties. Results. We find a trend of the fractional polarization with the cluster impact parameter, with fractional polarization increasing at the cluster periphery and decreasing toward the cluster center. Such trend can be reproduced by a magnetic field model with central value of few \u3bcG. The logrank statistical test indicates that there are no differences in the depolarization trend observed in cluster with and without radio halo, while the same test indicates significant differences when the depolarization trend of sources in clusters with and without cool core are compared. The comparison between clusters with high and low temperatures does not yields significant differences. Although the role of the gas density should be better accounted for, these results give important indications for models that require a role of the ICM magnetic field to explain the presence of cool core and radio halos in galaxy clusters. \ua9 2011 ESO

Measurements and simulation of Faraday rotation across the Coma radio relic

The aim of this work is to probe the magnetic field properties in relics and infall regions of galaxy clusters using Faraday rotation measures. We present Very Large Array multifrequency observations of seven sources in the region south-west of the Coma cluster, where the infalling group NGC 4839 and the relic 1253+275 are located. The Faraday rotation measure maps for the observed sources are derived and analysed to study the magnetic field in the south-west region of Coma. We discuss how to interpret the data by comparing observed and mock rotation measure maps that are produced simulating different three-dimensional magnetic field models. The magnetic field model that gives the best fit to the Coma central region underestimates the rotation measure in the south-west region by a factor of \u2dc6, and no significant jump in the rotation measure data is found at the position of the relic. We explore different possibilities to reconcile observed and mock rotation measure trends, and conclude that an amplification of the magnetic field along the south-west sector is the most plausible solution. Our data together with recent X-ray estimates of the gas density obtained with Suzaku suggest that a magnetic field amplification by a factor of \u2dc3 is required throughout the entire south-west region in order to reconcile real and mock rotation measure trends. The magnetic field in the relic region is inferred to be \u2dc 2 \u3bcG, consistent with inverse Compton limits

Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA

We investigate the possibility for the SKA to detect and study the magnetic fields in galaxy clus- ters and in the less dense environments surrounding them using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic field models for galaxy clusters of different masses and in different stages of their evolution, and derive mock rotation measure observations of back- ground radiogalaxies. According to our results, already in phase I, we will be able to infer the magnetic field properties in galaxy clusters as a function of the cluster mass, down to 1013 solar- masses. Moreover, using cosmological simulations to model the gas density, we have computed the expected rotation measure through shock-fronts that occur in the intra-cluster medium during cluster mergers. The enhancement in the rotation measure due to the density jump will permit to constraint the magnetic field strength and structure after the shock passage. SKA observations of polarised sources located behind galaxy clusters will answer several questions about the magnetic field strength and structure in galaxy clusters, and its evolution with cosmic time

Erratum: Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes (Astronomy and Astrophysics (2013) 552 (A58 ) DOI: 10.1051/0004-6361/201220728)

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